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ADS5545 Datasheet(PDF) 31 Page - Texas Instruments

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Part No. ADS5545
Description  11-BIT, 200 MSPS ADC
Download  54 Pages
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Maker  TI [Texas Instruments]
Homepage  http://www.ti.com
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ADS5545 Datasheet(HTML) 31 Page - Texas Instruments

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APPLICATION INFORMATION
THEORY OF OPERATION
ANALOG INPUT
Resr
200 W
Lpkg
6nH
10 W
Sampling
Capacitor
Csamp
2.4pF
INP
INM
Cbond
2pF
50 W
Cpar1
0.8pF
Cpar2
1pF
Ron
15 W
Ron
10 W
Ron
15 W
Cpar2
1pF
50 W
1.6pF
Lpkg
6nH
10 W
Cbond
2pF
Resr
200 W
Csamp
2.4pF
Sampling
Capacitor
Sampling
Switch
Sampling
Switch
R-C-RFilter
Drive Circuit Requirements
ADS5517
SLWS203 – DECEMBER 2007
ADS5517 is a low power 11-bit 200 MSPS pipeline ADC in a CMOS process. ADS5517 is based on switched
capacitor technology and runs off a single 3.3-V supply. The conversion process is initiated by a rising edge of
the external input clock. Once the signal is captured by the input sample and hold, the input sample is
sequentially converted by a series of lower resolution stages, with the outputs combined in a digital correction
logic block. At every clock edge, the sample propagates through the pipeline resulting in a data latency of 14
clock cycles. The output is available as 11-bit data, in DDR LVDS or CMOS and coded in either straight offset
binary or binary 2’s complement format.
The analog input consists of a switched-capacitor based differential sample and hold architecture, shown in
Figure 33.
This differential topology results in good ac-performance even for high input frequencies at high sampling rates.
The INP and INM pins have to be externally biased around a common-mode voltage of 1.5 V available on VCM
pin 13. For a full-scale differential input, each input pin INP, INM has to swing symmetrically between VCM +
0.5 V and VCM – 0.5 V, resulting in a 2-VPP differential input swing. The maximum swing is determined by the
internal reference voltages REFP (2.5 V nominal) and REFM (0.5 V, nominal).
Figure 33. Input Stage
The input sampling circuit has a high 3-dB bandwidth that extends up to 800 MHz (measured from the input pins
to the voltage across the sampling capacitors)
The input sampling circuit of the ADS5517 has a high 3-dB analog bandwidth of 800 MHz making it possible to
sample input signals up to very high frequencies. To get best performance, it is recommended to have an
external R-C-R filter across the input pins (Figure 34). This helps to filter the glitches due to the switching of the
sampling capacitors. The R-C-R filter has to be designed to provide adequate filtering (for good performance)
and at the same time ensure sufficient bandwidth over the desired frequency range.
In addition, it is recommended to have a 15-
Ω series resistor on each input line to damp out ringing caused by
the package parasitic. At higher input frequencies (> 100 MHz), a lower series resistance around 5
Ω to 10 Ω
should be used. It is also necessary to present low impedance (< 50
Ω) for the common-mode switching
currents. For example, this could be achieved by using two resistors from each input terminated to the
common-mode voltage (Vcm).
Copyright © 2007, Texas Instruments Incorporated
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